The thermodynamic efficiency of power generating cycles depends on the maximum temperature of its working fluid which, in the case for example of a gas turbine, is the hot gas exiting the combustor. The maximum feasible temperature of the hot gas is limited by combustion emissions as well as by the operating temperature limit of the metal parts in contact with this hot gas, and on the ability to cool these parts below the hot gas temperature. The cooling of the hot gas duct walls forming the hot gas flow paths of advanced heavy duty gas turbines is difficult and currently known cooling methods carry high performance penalties, i.e. lead to a reduction in power and efficiency.
Impingement cooling is one of the most effective cooling techniques for components which are exposed to gases with high hot gas temperatures. For impingement cooling of a duct a sleeve is disposed a short distance away from the duct outer surface. The impingement sleeve contains an array of holes through which compressed cooling gas discharge to generate an array of air jets which impinge on and cool the outer surface of the duct. After impingement the cooling gas flows in a cooling path delimited by the duct and the impingement sleeve towards one end of the duct. This flow leads to a so called cross flow. Usually the first impingement rows allow impingement of the duct without any cross-flow in the cooling channel. As the number of subsequent impingement rows is increasing towards the end of the duct, the cross flow in the cooling channel builds up. As a disadvantage, the increasing cross flow in the cooling channel hinders and lowers the possible heat transfer coefficients of the impingement cooling as the impingement jets are diverted and bent away from the combustion chamber wall (see FIG. 3a) before they impinge on it.
To limit the cross flow velocity it has been suggested in the U.S. Pat. No. 4,719,748 A to increase the height of the cooling channel over the length of the cooling channel. However, an increase of the height of the cooling channel reduces the speed of the impingement jet reaching the duct wall.
In addition to the decreasing efficiency of impingement cooling over the length of a duct cooled with impingement cooling the typical heat load of a duct wall is not homogeneous. For example most combustion chambers of gas turbines show an inclination with respect to the engine axis, which leads to a change in the hot gas flow direction. The hot gas flow in the combustion chamber has to adapt to this change in main flow direction leading to areas with higher heat load, so-called hot spots, on typical locations off the combustion chamber walls. To ensure the life time of the areas of the duct which are exposed to increased heat load as increased cooling is required at these locations.